Voltage drop on radial and ring

[PJW]

when working out voltage drop on a lighting circuit to work out the amp's (lb)I don't have a problem, say 10 lights @ 100 watts each 1000 watts 4.35 amps but what should I apply for a ring final and radial circuit.

 

[Octopus]

Would you install lights directly onto a ring final?

 

[Wilko]

You could try worst case - say for a 16A radial, 13A on last outlet and remainder on one back ?

 

[PEG]

Hi,are you confused as to what the acceptable VD is,on those circuits,or how to calculate,given layout and conductor size?

 

[PJW]

Hi,are you confused as to what the acceptable VD is,on those circuits,or how to calculate,given layout and conductor size?

I am try to work out the voltage drop, i have a radial circuit with 5 double 13amp sockets i just need to work out lb

 

[telectrix]

assuming you are using a 20A MCB for the circuit, then Ib must be less than 20A. ( Ib<In<Iz). if you estimated load is more, then you need to redesign the circuit.

 

[Richard Burns]

To calculate the volt drop on a circuit the basic approach is to assume the worst case of the total load is entirely at the end of circuit for a radial or at the midpoint on a ring then you calculate your
VD= mV/A/M x ib x L / 1000 for a radial or (mV/A/M x ib x L / 1000) / 4 for a ring and ensure that this is not greater than the specification standard for the loads or is within 3% for lighting or 5% for non lighting of the supply voltage
If you need to calculate more precisely then you need to know the specific loads at each point and the lengths of cable for each section where the current is different, calculate the volt drop for each section and add the values for the sections to get a total volt drop (more difficult on a ring) for the circuit.

Just seen the last post so too slow typing.
Volt drop on a socket circuit cannot be calculated precisely as the loads may change drastically so follow Wilko's advice or assume the Ib is the rating of the OCPD at the extremity of the circuit barring any other information that provides a design current.
A ring is generally considered to carry 20A at the mid point and 12A at other points in the circuit; equivalent to 26A at the midpoint.

 

[Ian1981]

Lights
Max volt drop allowed is 6.9 volts (3% of 230volts) x 1000
Divide by
Ib/2 x mV/a/m

Sockets volt drop
4x 11.5 volts x1000
Divide by
Ib x mV/a/m

Will dive max length allowed before volt drop is exceeded

 

[PJW]

assuming you are using a 20A MCB for the circuit, then Ib must be less than 20A. ( Ib<In<Iz). if you estimated load is more, then you need to redesign the circuit.

What I'm trying to work out is the lb of 5 13amp sockets, I know how to work it out in a lighting circuit say 10 lights, 10 X 100 watts / 230 = 4.34 so lb of 4.34

 

[telectrix]

What I'm trying to work out is the lb of 5 13amp sockets, I know how to work it out in a lighting circuit say 10 lights, 10 X 100 watts / 230 = 4.34 so lb of 4.34

yes but you probably won't have all the lights on at the same time, so some diversity can be allowed in a domestic..... comml. and indl. may require all on simultaneously. same with socket outlets. you can never be precise in determining the IB unless you know exactly what is going to be plugged in, which is impossible.

 

[PJW]

yes but you probably won't have all the lights on at the same time, so some diversity can be allowed in a domestic..... comml. and indl. may require all on simultaneously. same with socket outlets. you can never be precise in determining the IB unless you know exactly what is going to be plugged in, which is impossible.

So just follow the rules set out in appendix 15 of BS7671 100m2 for ring final and 50m2 for radial if using a 20A CB

 

[Richard Burns]

If you have five 13A sockets then you would need to know the rating of the breaker protecting them and use that as your limiting design current.
For a ring this would be 32A, for a radial it could be from (normally) 16 to 32A.

If you look in the tables in the on site guide chapter seven, this gives standard circuits and maximum lengths of cable. In most cases volt drop is the limiting factor (see the notes at the end of the table). This shows you the maximum length of a ring final circuit and various radial circuits, normally limited by volt drop.

 

[PJW]

If you have five 13A sockets then you would need to know the rating of the breaker protecting them and use that as your limiting design current.
For a ring this would be 32A, for a radial it could be from (normally) 16 to 32A.

If you look in the tables in the on site guide chapter seven, this gives standard circuits and maximum lengths of cable. In most cases volt drop is the limiting factor (see the notes at the end of the table). This shows you the maximum length of a ring final circuit and various radial circuits, normally limited by volt drop.

Yes I've seen this but I'm using a 60898 breaker and that's not listed

 

[PEG]

Hi,that is the type,not the size,unless i have missed something...

 

[Richard Burns]

Yes I've seen this but I'm using a 60898 breaker and that's not listed

You are using a circuit breaker i.e a cb in the tables, wherever it states cb/RCBO this means either a circuit breaker (to BSEN60898) or an RCBO (to BSEN61009) of the type listed.
Once you know the current rating of your 60898 circuit breaker then you can look up the maximum length in the table.

 

[Ian1981]

It should be bore in mind that the tables assume the maximum voltage drop is being applied to the listed circuits